End seal for transmission lines



y 1945- L. w. RICHARDSON EI AL 2,376,725

END SEAL FQR TRANSMISSION LINES I Filed Feb. 28, 1942 Lycurgus W.RichardsomDecease'd, Marie Maurer Richardson, Administratr'ix,

Marvel WScheldorf,

b WW6. XLAJW Their Attorney.

Patented May 22, 1945 END SEAL FOR TRANSMISSION LINES Lycurgus W.Richardson, deceased, late of Scotia, N. Y., by Marie Maurer Richardson,administratrix, Scotia, N. Y., and Marvel W. Scheldorf,

Schenectady, N. Y., assignors to General Electrlc Company, a corporationof New York Application February 28, 1942, Serial No.'432,844

2 Claims. 174-19) This invention relates to transmission lines, and moreparticularly to mechanical connections between the two conductorsthereof.

Conductors which are utilized for the transmission of high frequencywaves are generally used in pairs spaced relatively closely to minimizeradiation therefrom. Spacing between such pairs of conductors, and thesizes of the individual conductors, determine certain transmissioncharacteristics of the transmission line, one of which is known as thesurge impedance, or characteristic impedance. Abrupt changes in suchimpedance along a transmission line generally produce deleteriouseffects, as is well known in the art. When two such conductors arespaced closely together, it is therefore necessary to use insulatingmeans between the conductors to maintain proper spacing therebetweenunder the effect of external forces.

It is an object of the invention to provide new and improved meanswhereby the two conductors of such a transmission line may be maintainedin proper spaced relation with minimum variation in impedance along theline.

It is a further object of the invention to provide such means which isespecially useful for maintaining the proper spacing between two suchconductors at the ends thereof.

It is an additional object of the invention to provide such spacingmeans which; are rugged and highly resistant to th effect of externalforces and the elements, and which is at the same time simple and easilyconstructed and assembled. I

The invention is particularly useful with coaxial transmission lines, inwhich the spacing means may be utilized for closing the passage betweenconductors against the passage of gas under pressure, used forinsulation purposes, and against moisture and the like. It isaccordingly also an object of the invention to provide such insulatingmeans, particularly useful in coaxial transmission lines, Whoseconfiguration is such, in conjunction with the coaxial lines, as to makethe total length of each line, including its end connections toassociated apparatus, substantially equal to the total length of theother line.

The features of the invention which are believed to be novel are setforth with particularity in the appended claims. The invention itself.both as to its organization and manner of operation. together withfurther objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing, in which as Fig. 1 of the drawing is a sectionalview along the axis of a transmission line embodying the invention, andFig. 2 is a modification thereof.

In Fig. 1 a coaxial transmission line including an outer cylindricalconductor in and an inner cylindrical conductor I I is terminated near agenerally disc-shaped insulator l2. A threaded member liextends throughthe insulator II and has a plug-shaped end It soldered or brazed withinthe end of the inner conductor H. A metal washer I5 is placed over thethreaded member l3 adjacent the plug l4 and firmly holds a disc I8 ofpacking material within the recess 16a in the insulator II. The disc I6is preferably made of synthetic rubber, as for example, polyisobutylenecompound, which is soft enough to spread and make a gas-tight sealbetween the center conductor II and the insulator I2. At the same timesuch material has long life when subjected to high frequency currents,light, oil, heat and most gases.

On the outside of the insulator l2 a disc ll, of some soft material suchas pure aluminum or annealed copper, is placed over the threaded memberl3, followed by a metal washer 08, a lock washer l9, and a nut 20 whichis screwed on the threaded member l3 to compress the insulator l2 andthe discs l6 and I1 tightly between the plug l4 and the nut 20. Due totheir softness, the discs l6 and I1 distribute pressure uniformly overthe central surface of the insulator i2, and minimize the chances ofbreakage. A second nut 2| is provided for convenience in making electricconnection through the threaded member l3 to the center conductor 5 I.

The insulator H, as is clearly shown in the cross-sectional view thereofis made somewhat dome-shaped, the under side of the dome facing theinterior of the transmission line. The outer part of the under face ofthe domed insulator is made flat in a plane perpendicular to theconductor ID to form a circular sealing surface 3 2.

The cylindrical outer conductor I0 is spun at its end portion near theinsulator l2 into a conical configuration 30 and the extreme end of theconed portion 30 so formed is further spun into a flange Si in a planesubstantially perpendicular to the conductors Ill and II. The flange 3|is shaped thus parallel to surface 32' to conform thereto and providegood gas sealing therebetween. This flange 3| and surface 32 may, ifdesired, take other mutually conforming shapes. A ring 33 of syntheticrubber, or other suitable material, is placed between the flange 3| andthe sealing surface 32 of the insulator l2, and a metal clamping ring Nis pressed toward the insulator l2 byscrews 35 suitably spacedtherearound and extending through the ring 34, the flange 3|, the ring33, and into threaded holes in the insulator l2.

, The dome shape of the insulator i2 provides sumcient mechanicalstrength so that gas under pressure may be confined between theconductors I. and II to increase the breakdown voltage therebetween, andalso to prevent breaking of the line with the consequent introduction ofmoisture.

the rubber disc It provide sufllcient sealing between the insulator l2and the conductors Hi and II so that such gas under pressure suffers noleakage.

The use of the insulator 12 provides a dielectric path between the screwmember II and the flange II, which path is sumciently long to withstandthe voltage between conductors in and II without breakdown, and due tothe shape of the insulator I! there is less capacity between theconductors It and H at the insulator than would be the case if the usualtype of insulator were provided between the conductors l and II withoutaltering their sizes. Although the use of this long ceramic path betweenthe flange 3| and the screw member It tends to minimize this capacity,yet this capacity is still larger than the capacity existing in asimilar length of the transmission line between the conductors in and IIwhere no insulator is present. If suitable provision were not made forcounteracting the effect of this added capacity between the conductorsill and II, there would be a discontinuity in the transmission line,with consequent deleterious eflects.

In the section of the transmission line including the coned portion 30of the outer conductor ID, the two conductors l0 and II are spacedfarther apart than at other sections of the trans mission line. Due tothe fact that they are so spaced, there is less capacity between theconed portion 30 of the outer conductor l0 and the inner conductor Hthan there is between other portions of the outer conductor l0 and theinner conductor I i. At the same time, the space en- Furthermore, therubber sealing ring 33 and v closed by the conductors l0 and II in thatsection of the transmission line including the conical section 30 isgreater than the space enclosed by a similar length of the transmissionline in which the conductor I0 is uniform in diameter. Due to the factthat the conductors l0 and II enclose such additional space at theconical portion 30, the inductance of the transmission line is greaterat this conical portion 30 than at other portions of the line.

At each incremental section of the coned portion 30 of the transmissionline there is a particular characteristic impedance, which-varies from avalue equal to that of the transmission line where the coned portion 30is joined to the uniform portion of the transmission line to a highersurge impedance at the open end of the coned portion 30. Due to the factthat this coned portion 30 thus has a smoothly varying characteristicimpedance, it tends to act as an impedance transformer.

When a particular material is used for the insulator l2 and a particularshape therefor is utilized, the amount of capacitance introduced betweenthe flange 3| of the outer conductor Iiiand the threaded member II isfixed. This does not mean that the capacity between the coned portion 30and the inner conductor H is fixed,

insulator l2. small, the coned portion 3! becomes very long,

for that capacity depends upon the angle to which the coned portion 30is flared. If this angle, formed by the inner surface of the conedportion "with the axis of the transmission line is too great, there isincreased capacity between the coned portion at through the insulator l2and the threaded member I3. I! this angle be too small, the inductanceadded by the flaring of. the coned portion 30 may be more than enough tocompensate for capacity effect caused by the Furthermore, if this angleis too and may approach a length equal to a substantial portion of awave length of a wave transmitted through the transmission line. It isnot desirable that the length of the coned portion 30 should be sogreat.

In any particular situation, where the dimensions of the inner and outerconductors II and III, and the shape and material of the insulator I!are chosen, a suitable choice of the angle made by the coned section 30with the axis of the transmission line may be made, So that theprogressive simultaneous decrease in capacity and increase in inductanceof the coned portion 30, progressing from the transmission line towardthe insulator l2, may be made effective just to counteract the changeinsurge impedance of the transmission line at the insulator l2 due to theincreased capacity between .the conductors l0 and II through theinsulator i2.

Although the optimum angle between the inner surface of the conicalportion 30 and the axis of. the transmission line generally liessomewhere between 20 and 60 degrees in any particular case, this angledepends entirely on the particular structure utilized for the insulatorl2 and the transmission line I, II. The dimensions of the transmissionline III, II are generally fixed by considerations not mentioned herein,such as the characteristic impedance of the source of oscillations, orthe load therefor. It is generally desirable to make the insulator l2 asthin as possible to reduce capacitance therethrough, and

yet thick enough to have sufflcient strength to maintain proper spacingbetween the conductors l0 and II and to resist the pressure of gas inthe transmission line.

It is generally desirable that the overall length of the conical portion30 and the insulator 12 be a small portion of a wave length of a wavetransmitted through the transmission line l0, II. If the particularstructure chosen for the insulator l2 be such as to'require a very smallangle between the inner surface of the coned portion 30 and the axis ofthe transmission line, thereby making the coned portion 30 ofconsiderable length with respect to a wave length, it is generallydesirable to change the formation, and possibly the material, of theinsulator If in such fashion that the angle between the inner surface ofthe coned portion 30 and the axis of the transmission line may beincreased substantially.

When the invention is used in an end connection for a coaxialtransmission line, as illustrated in Fig. 1, especially advantageousconnections may be made to desired apparatus. Such connections are madeby means of a short transmission line comprising two parallel conductorswithout introducing large amounts of inductance, and consequentdiscontinuity, at any part of the coaxial transmission line or of theparallel conductor connections. In Fig, 1 a wide flat conductor 40 isfastened to the conducting ring 34 by means of a screw I, and a parallelwide, flat conductor 42 is fastened upon the threaded member l3 by meansof the nut 2|. If desired, the characteristic impedance between the wideflat conductors 4e and 42 may be made the same as the characteristicimpedance between the conductors ill and l i. To this end, the spacingbetween the conductors 40 and 42, and the width of these conductors, maybe given such values that there is any desired characteristic impedancetherebetween. It should be noted particularly that the conductor 45extends only a small distance beyond the end of the conductor 42, sothat no substantial extra inductance is introduced between theconductors 45 and 42, as is the case in connections made with endterminals provided previously for coaxial transmission lines.

In Fig. 2 a connecting arrangement is shown in which a connection ismade between a coaxial transmission line including the conductors illand II and a second similar coaxial transmission line including an outerconductor 50 and an inner conductor 5|. Certain elements of the deviceof Fig. 2 are like those of the arrangement illustrated in Fig. 1, andare designated by like reference numerals. A flat insulating disc-52 isinterposed between the two coaxial transmission lines, the flat flangedportion 3| of the outer conductor bearing through washer 33 on one sideof the disc 52, and a similar flat flanged portion 53 of the outerconductor 56 bearing through a washer 54 against the other side of theinsulating disc 52.

The outer conductors I0 and 50 of the two transmission lines are heldtogether by 'bolts 55 extending through the conducting ring 34, flange 3I, washer 33, insulating disc 52, washer 54, flange 53, and anotherconducting ring 56 similar to conducting ring 34.

The threaded member l3 at the end of the inner conductor H extendsthrough thecenter of the insulating disc 52 and through washers 51 and58, respectively, similar to washers l6 and I5, into a metal plug 59fastened into the end of the inner conductor The threaded member 13 isscrewed tightly into the plug 59 to maintain a flrm'electricalconnection between the inner conductors l l and 5|.

A hole Gll is provided through the insulating disc 52 to allow for thepassage of fluid therethrough. Where gas under pressure is maintainedbetween the inner conductors I l and 5! and the outer conductors l0 and50, the pressure of the gas in various sections of the line is equalizedthrough the hole 60. There may, if desired, be more than one of theholes 60, as for example, three such holes.

The electrical structure of the joint including the insulating disc 52between the twocoaxial transmission lines in Fig. 2 is similar to thatof the arrangement of Fig. 1. The disc 52 is of larger diameter than theouter conductors i0 and 50, thereby minimizing the capacitance addedbetween the inner and outer conductors of the two transmission lines.This added capacitance is compensated by the inductance added betweenthe inner and outer conductors of the two transmission lines by theconical portion 35 of the outer conductor l0 and by a similar conicalportion 6| of the outer conductor 50.

Where, as in the arrangement of Fig. 2, there are two conical portions30 and it, each of which introduces added inductance into its coaxialtransmission line, the inductance added by each of these conicalportions 30 and 5! need not be so great as the inductance added by thesingle conical portion in the arrangement of Fig. 1. Generally,therefore, the conical portion 3a in the arrangement of Fig. 2 need notbe as long as the conical portion 35 in the arrangement of Fig. 1.

While we have shown and described a particular embodiment of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and we, therefore, aim in the appended claims tocover all such changes and modifications as fall within the true spiritand scope of the invention.

What we claim as new and desire to secure by Letters Patent in theUnited States is:

1. An end seal for a high frequency transmission line of the type havinga tubular outer conductor and a coaxially disposed inner conductorcomprising, a disc of insulating material afixed centrally to one end ofsaid inner conductor and having a diameter larger than the diameter ofsaid outer conductor ,a section of said outer conductor at the endthereof adjacent said one end of said inner conductor being flaredoutwardly at an angle to the periphery of said disc and being affixedthereto, said flared section of said outer conductor having a. lengthsubstantially smaller than the length of a wave transmitted by saidtransmission line, the angle of said flared section being such that theincreased inductance introduced between said conductors by said flaredsection is sufiicient to compensate substantially completely for theincreased capacity introduced therebetween by said insulating disc, saiddisc being of a thin domelike shape to afiord maximum mechanicalstrength to resist mechanical forces between said conductors.

2. An end seal for a gas filled high frequency transmission line of thetype having a tubular outer conductor and a coaxially disposed innerconductor comprising, a disc of insulating material aflixed centrally toone end of said inner conductor and having a diameter larger than thediameter of said outer conductor, a section of said outer conductor atthe end thereof adjacent said one end of said inner conductor beingflared outwardly at an angle between 20 and to the periphery of saiddisc and being atflxed thereto, said flared section of said outerconductor having a length substantially smaller than the length of awave transmitted by said transmission line, the angle of said flaredsection being such that the increased inductance introduced between saidconductors by said flared section'is suflicient to compensatesubstantially completely for the increased capacity introducedtherebetween by said insulating disc, and fluid sealing means interposedbetween said flared section and said disc and between said innerconductor and said disc, said disc being of a thin domelike shape toafford maximum mechanical strength to resistmechanical forces betweensaid conductors and the pressure of said gas within said line.

MARIE MAURER. RICHARDSON, Administratria: of the Estate of Lycurgus W.Richardson, Deceased.

MARVEL W. SCHELDORF.

